With higher levels of integrated circuits on semiconductor chips and the need for faster transistors in these circuits, the FET transistor must maximize all aspects of semiconductor physics to fabricate transistors in these circuits with faster switching speed.
In crystalline solids, such as monocrystalline silicon, the atoms which make up the solid are spatially arranged in a periodic fashion. This periodic arrangement of atoms in a crystal is called a lattice. The crystal lattice always contains a volume which is representative of the entire lattice and it is regularly repeated throughout the crystal. The directions in a lattice are expressed as a set of three integers with the same relationship as the components of a vector in that direction. The three vector components are given in multiples of the basic vectors. For example, in cubic lattices, such as silicon which has a diamond crystal lattice, the body diagonal has the components of 1a, 1b, and 1c and this diagonal exist along the [111] direction with the [ ] brackets being used to denote a specific direction. However, many directions in a crystal are equivalent, depending on the arbitrary choice of orientation of the axes. Such equivalent directions are denoted with &lt; &gt; brackets and, for example, crystal directions in the cubic lattice [100], [010], and [001] are all crystallographically equivalent and are &lt;100&gt; directions. Since these directions will also be on the negative side of the origin, as arbitrarily defined, they also are identified with a (-) over the specific negative integer, such as [100], [010], and [001] for &lt;100&gt; directions. Unless specifically stated or shown in the following description in this application, a crystal direction includes both positive and negative integers.
Planes in a crystal also can be determined with a set of three integers h, k, and l. They are used to define a set of parallel planes and each set of three integers in () parentheses identify a specific plane. As in the case of directions, many planes in a lattice are equivalent and the indices of such equivalent planes are denoted by { } parentheses. For cubic lattices, direction [k,l,m] is perpendicular to a plane with the identical three integers (k,l,m). Thus, if either a direction or a plane of a cubic lattice is known, its perpendicular counterpart can be quickly determined without calculation. For example, for planes of equivalent symmetry such as {100} plane, the equivalent planes are (100), (010), (001), (100), (010), and (001). Like the crystal direction, the crystal plane in the following description in this application includes both positive and negative integers unless specifically stated otherwise.
Today, almost all FET transistors are fabricated using {100} silicon wafers because the smallest surface state densities are present with such crystal orientations. The {100} crystal orientations is found using x-ray to examine a boule of silicon and to produce a Laue photograph. With this information, a primary flat is ground on the silicon boule to precisely identify the {100} crystal orientations. After the boule is sliced into thin wafers, the flat is used for establishing the {100} crystal orientation during the fabrication of integrated circuits, including FETs, in the individual chips after the wafer is diced. Each wafer is etched with an identification number which includes the dopant species and the crystal growth orientation. In manufacturing the FET with the source and drain separated by the gate and having a channel thereunder, the flat permits alignment of the wafer relative to the exposure masks so that the gate width and the channel length thereunder will be parallel with [100] crystal direction and orthogonal with (100) crystal plane.
Another crystal direction in the {100} silicon wafer is [110] and it is known to have higher mobility than a [100] crystal directions in the {100} silicon wafer. When an electric field is applied to semiconductor body such as silicon, each of the carriers, such as electrons, in the body will experience a force from the field and will be accelerated along the field in the opposite direction of the field. This is called drift velocity and it is proportional to the applied electric field. This proportionality factor is known as mobility and it varies based on a number of factors including the crystal direction of the semiconductor body, such as silicon. From experiments, it has been found that the mobility of the carriers in {100} silicon in the [110] direction is higher than the [100] direction by about 5 percent depending on the doping concentration of the silicon. However, silicon in [100] crystal direction has better cleavage than silicon in [110] crystal direction and breaks cleaner along scribe lines so that the chips do not fracture during dicing. Thus, for the ease in manufacturing, it has been the preferred crystal direction in the fabrication of integrated circuits in silicon wafers.